Gasteropelecuslevis(EIGENMANN, 1909)

Silver Hatchetfish

Etymology

Gasteropelecus: Greek gaster [= stomach] and pelecus [=hatchet, axe]

levis: Latin for smooth. According to his notes, Eigenmann placed this species in his new genus Pterodiscus solely on the basis of the lack of maxillary teeth; some sources suggest this is the “smooth” reference.

Habitat

Maximum Standard Length

An aquarium with a surface area measuring a minimum of 80 ∗ 30 cm or equivalent is required.

Maintenance

Cover a good portion of the water surface with floating plants, which will help to make this flighty species less skittish. It also prefers a bit of current against which to swim. Other decor is not particularly critical, but it looks good in both heavily planted set-ups and Amazon biotope-style tanks, with driftwood branches, a sand substrate and some leaf litter. Make sure the tank cover is very tightly fitting, as it is capable of clearing several metres in a single jump.

Water Conditions

Diet

Naturally feeds on insects on and probably above the surface, crustaceans, worms. A study carried out on Thoracocharax stellatus (Netto-Ferreira et al, 2007) determined that 99.6% of the diet consisted of insects, and the majority (87.6%) of these were terrestrial.

Sometimes a little reluctant to take dried foods initially, but it will usually learn to accept them over time. A large proportion of the diet should consist of frozen and live foods, such as bloodworm and Daphnia. Gut-loaded Drosophila fruit flies make an excellent food if available.

Very peaceful but it can be nervous and shy, and will not compete well with boisterous species for food. Good tankmates include other small fishes that inhabit different parts of the tank, for example tetras and dwarf cichlids, or catfishes such as Corydoras and smaller Loricariids. Always buy at least half a dozen, as it will not settle without the security of conspecifics.

Sexual Dimorphism

Females are noticeably rounder-bodied than males when full of eggs.

Reproduction

This species spends almost all of its time at or just below the water surface, although it will sometimes retreat into midwater if threatened or feeding. Like other freshwater hatchetfishes, it is renowned for its ability to leap from the water surface and glide for distances of several metres. This behaviour is used both to catch flying insects, and to escape potential predators.

According to the initial description by Eigenmann, this species (unlike others in the genus) has no teeth on the maxillary, and for this reason he erected the genus Pterodiscus [derived from the Greek meaning winged disc] for it. Weitzman (1960) placed the species in Gasteropelecus and invalidated Pterodiscus. [There is a botanical genus Pterodiscus, erected by Linnaeus in 1753.]

The family Gasteropelecidae is separated from other Characiformes by the following combination of characters: frontal bone longitudinally corrugated with a strong longitudinal ridge; posttemporal and supracleithrum fused into a single bone; pelvic fins and associated bones minute; presence of an enlarged, strongly convex muscular pectoral girdle region, consisting of greatly expanded coracoids fused to a single fan-shaped and corrugated median bone; lateral line extending ventro-posteriorly to approach the anal-fin insertion; 0-2 or 3 scales behind head and one or very few scales on caudal-fin base; 10-17 dorsal-fin rays; 22-44 anal-fin rays; adipose-fin present in larger species (those in Gasteropelecus and Thoracocharax), absent in smaller species (those in Carnegiella).

It contains three genera. Thoracocharax, the most primitive and distinguished by its impressive keel, contains two species, T. securis and T. stellatus. Gasteropelecus contains three species,G. sternicla, G. levis and G. maculatus. These two genera contain the largest in size of the hatchetfish, and all are silver in colouration; G. sternicla is the more frequently seen of these five species, though any of them may be offered as “Silver Hatchetfish.” The third genus, Carnegiella, contains four described species that are the most derived or specialized of the hatchetfishes, and all are smaller and lack an adipose fin.

Gasteropelecids are commonly-referred to as ‘freshwater hatchetfishes’ due to their heavily-keeled body shape which has evolved in such a way due to possessing an enlarged, heavily-muscled pectoral girdle, and which resembles the shape of a hatchet head.

They are sometimes said to be capable of propelled flight above the water surface by beating their pectoral fins but in fact this is not the case and was disproven by Wiest (1995) who used high speed video photography to demonstrate that the pectoral fins are used in leaving the water but not while the fish is in the air. The action is more accurately described as a powerful jump and is actually a modified threat response which only appears to be used in extreme circumstances much like in many other fishes.

The researcher also found that the fish only seem to be able to jump once or twice before requiring a rest period due to the large amount of energy required to work the massive pectoral girdle muscles, and when fatigued their threat response is comparable to that of other small species in that they dive towards the substrate.

In addition anecdotal evidence from fish collectors and icthyologists working in the field would suggest that gasteropelecids rarely jump and they also tend not to do so in aquaria, even when being chased with a net (pers. obv.). However, leaving the tank cover open during darkness is likely to result in one or more hatchetfish on the floor by morning, so clearly they can and will jump.

This hatchetfish can be quite delicate when initially imported, but once acclimatised proves to be a very good aquarium resident. It’s often confused with the common hatchet, G. sternicla, which confusingly is the species usually labelled as the silver hatchetfish in shops.

Characiformes is among the most diverse orders of freshwater fishes currently including close to 2000 valid species distributed among 19 families. This tremendous taxonomical and morphological diversity has historically impaired the ability of researchers to resolve their genetic relationships with many genera remaining incertae sedis. A further limiting factor has been that in many cases exhaustive study of these on an individual basis is the only way to resolve such problems. Modern molecular phylogenetic techniques have allowed some headway, though, and research papers by Javonillo et al. and Oliveira et al., published in 2010 and 2011, respectively, revealed some interesting hypotheses.

The results of the former suggest that Gasteropelecidae forms a monophyletic clade nested within the family Characidae, while the latter expanded on this and concluded that within the family Carnegiella is the sister group of Gasteropelecus with Thoracocharax sister to that clade, and that Gasteropelecidae appears most closely-related to the authors’ expanded Bryconidae which contains the genera Brycon, Henochilus and Salminus.

Others such as Oliveira et al. (2011) have concluded that the family Erythrinidae is also closely-related to this grouping with Hepsetidae and Alestidae more distant.

References

Eigenmann, C. H., 1909 - Annals of the Carnegie Museum v. 6 (no. 1): 4-54Reports on the expedition to British Guiana of the Indiana University and the Carnegie Museum, 1908. Report no. 1. Some new genera and species of fishes from British Guiana.

I have owned one silver hatchetfish, and he recently died due to jumping out of the aquarium. He was a young one and there was only a tiny gap for him to jump out of. If you are interested in owning one of these guys, make sure you have a good lid.